transformation 
affin polynomial (3. degree) 
picture 887 27.3 um 26.2 um 
Table 1 : Mean square residuals for the transformation of the control 
points 
3.3 Preparation of a reference coordinate system 
Photogrammetric calculations are based on cartesian coordinate systems. 
But in the present case the available pass poînts and the DTM control 
information were defined in the system of the UTM projection and in the 
ellipsoidal WGS72 coordinate system respectively. Therefore a procedure 
has to be established to transform the coordinates into a local carte- 
sian system with its origin in the center of the model area (Qp) and the 
Xy-plane normal to the international ellipsoid (ef. fis. 2). 
$ Ellipsoid- 
3.E normale 
  
  
Figure 2: Definition of a local cartesian coordinate system 
The transformation for the pass points then looks as follows: 
UTM => ellipsoid => geocentric system -> local cartesian system 
For control of the correlation results a digital terrain model was 
available. This data, defined in WGS72, covered an area of 1° lattitude 
by 1° longitude with 3.600 points in each direction. In total the DTM 
provides 1,296 million points with a point distance of about 20m in 
longitude and 30m in lattitude direction. The heights were derived by 
digization of topographic maps with an accuracy of about 3-8 m, what is 
good enough for the examination of the correlation results. 
The comparision of the DTM-heights with the correlated ones will be 
accomplished in the ellipsoidal system. That makes two transformations 
necessary. First, the corner points of a selected part of the DTM have 
to be projected into the local system to control the point determination 
during correlation and then the resulting heights will be transformed 
back onto the ellipsoid. 
The current dimensions of the DTM allow to compare equally spaced raster 
points defined in the cartesian coordinate system with an ellipsoidal 
raster having equal angular steps (cf. fig.3). Equation 1 expresses the 
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